(1) Field of the Invention
The present invention relates to an actuating control method of a thermal head which controls time period for turning on electricity to a thermal head in order to guarantee an excellent printing operation.
(2) Description of the Prior Art
The thermal head has a heating elements' row, in which a plurality of heating elements are disposed on a substrate in a straight line. And, the thermal head is driven by selectively turning on electricity to each heating element. In such a thermal head, since the temperatures of each heating element and the substrate increase during the printing operation due to its structural nature, the time period for turning on electricity to each heating element is necessary to be corrected by taking the increase of temperatures of each heating element and the substrate into consideration so as not to cause uneven printing finish in the printing operation.
For this correction of the time period for turning-on electricity, there has been known a method in which the correction is carried out in each heating element based on the heat accumulation amount in the periphery of each heating element or a method in which the correction is totally carried out based on the overall heat accumulation amount around the substrate as a whole.
In the case that the correction is totally carried out based on the overall heat accumulation, however, a conventional actuating control method for a thermal head was not performed by taking the position of each heating element into consideration. That is, the conventional actuating control method was carried out by merely correcting the time period for turning on electricity commonly with respect to each heating element based on the sum of previous history of turning-on of electricity to each heating element.
In the case that even if the frequency for turning on electricity to each heating element is different in the respective heating element, for example, the condition of turning-on of electricity is extremely different between the upper part and the lower part in the heating elements' row, the control is uniformly carried out with respect to each heating element. Accordingly, there was a problem such that the control cannot be carried out adequately so as to response to an actual heat accumulation amount in each heating element, thereby resulting in generation of uneven printing finish in the printing operation. And, therefore, it could not be possible to realize the printing operation guaranteeing uniform printing density.
Further, the heat accumulation in the thermal head is usually influenced with the configuration or material or printing pattern. Therefore, the edge part of the heating elements' row is likely to be cooled down easily than the central part thereof. Thus, the heat accumulation amount is generally larger in the central part than the edge part.
If the correction of turning-on time period of electricity is commonly controlled with respect to all the heating elements without taking their positions into consideration, the heat accumulation difference causes undesirable printing density difference in the printing operation. As a result, there was a problem such that the high quality printing cannot be realized.